1. Field of the Invention
The present invention relates to a rechargeable lithium battery, and more particularly to a rechargeable lithium battery which utilizes the improved active material for its positive or negative electrode.
2. Description of Related Art
In recent years, rechargeable lithium batteries have been extensively developed. The performance characteristics of rechargeable batteries, such as charge-discharge voltages, charge-discharge cycle life characteristics and storage capabilities, depend largely on the particular electrode active material used. For example, the use of sulfide-based positive active material, such as TiS2, is known to result in the reduction of battery voltages. This is attributed to the presence therein of free sulfur which reacts with a negative electrode. In order to overcome this deficiency, Japanese Patent Laid Open No. Sho 60-175371 (1985) proposes a method whereby a metal powder showing a tendency to readily react with sulfur, such as a copper powder, is added to the positive electrode.
However, the use of TiS2 for the positive active material has imposed a problem of resulting in insufficient charge-discharge cycle characteristics (See, for example, Lawrence P. Klemann, J. Electrochem. Soc., Vol.128, No.1, 1981, pp. 13-18).
The present invention is directed toward solving the above-described problems and its object is to provide a rechargeable lithium battery which exhibits improved charge-discharge cycle characteristics.
The rechargeable lithium battery of the present invention has a positive electrode, a negative electrode and a non-aqueous electrolyte. Characteristically, the positive or negative electrode contains, as its active material, a complex sulfide represented by the compositional formula MxTi1xe2x88x92xSy, wherein M is selected from at least one of Cu, Zn, Cr, Mn, Co and Ni, x satisfies the relationship 0 less than xxe2x89xa60.18, and y satisfies the relationship 1.65xe2x89xa6yxe2x89xa62.25. The complex sulfide may further contain lithium.
In accordance with the present invention, the inclusion of the metallic element M (at least one of Cu, Zn, Cr, Mn, Co and Ni) in the crystal lattice of titanium complex oxide results in stabilizing the crystal structure of the active material. The use of titanium complex oxide for the active material of the positive or negative electrode thus leads to the improved charge-discharge cycle characteristics of resulting rechargeable lithium batteries.
Any of the above-listed metallic elements M for use in the present invention is known to form a stable compound with sulfur (S) and have a decomposition temperature of not below 1,000xc2x0 C. (See, for example, binary phase diagrams for M-S in xe2x80x9cBinary Alloy Phase Diagramsxe2x80x9d, American Society for Metals, 1986). Since any of these metallic elements M tends to be chemically bound to sulfur by a relatively strong force, such a metallic element M is believed to occupy certain sites in a crystal lattice of the TiS2 phase and thus stabilize its crystal structure. Accordingly, other elements which also form compounds with sulfur, e.g., Cd, In, La, Ce, Sm, W and Pt, when introduced into titanium complex oxides to form solid solutions, are expected to be also effective in improving charge-discharge cycle life performance characteristics.
In the present invention, the stoichiometry x of the metallic element M in the above-specified complex sulfide composition is maintained not to exceed 0.18. If the inclusion of the metallic element M is excessive, i.e., if x exceeds 0.18, a simple substance or sulfide phase composed principally of M may be deposited to result in lowering the improving effect of charge-discharge characteristics.
In the present invention, the aforementioned complex sulfide for use as the positive or negative active material has a layered crystal structure similar to that of TiS2. The presence of such a crystal structure can be identified by X-ray diffraction (XRD).
An electrolyte solvent for use in the rechargeable lithium battery according to the present invention can be selected from non-aqueous electrolyte solvents generally employed for rechargeable lithium batteries. Specifically, it may be a mixed solvent of cyclic carbonate and chain carbonate, for example. Examples of cyclic carbonates include ethylene carbonate, propylene carbonate and butylene carbonate. Examples of chain carbonates include dimethyl carbonate, methyl ethyl carbonate and diethyl carbonate. The electrolyte solvent may alternatively be a mixed solvent of the aforestated cyclic carbonate and an ether solvent, for example. Examples of ether solvents include 1,2-dimethoxyethane, 1,2-diethoxyethane and the like. Examples of useful electrolyte solutes include LiPF6, LiBF4, LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(CF3SO2) (C4F9SO2) LiC(C2F5SO2)3 and any combination thereof, for example. Other applicable electrolytes include gelled polymer electrolytes wherein a liquid electrolyte is impregnated in polymers such as polyethylene oxide and polyacrylonitrile, and inorganic solid electrolytes such as LiI and Li3N, for example.
In the present invention, any non-aqueous electrolyte can be used, so long as it contains an Li compound as a solute for realizing an ionic conductivity, and a solvent used to dissolve and hold the solute is hardly decomposed at voltages during battery charge, discharge and storage.
In the case where the aforementioned titanium complex sulfide is used for the positive active material, a suitable negative active material may be chosen from carbon materials capable of electrochemical storage and release of Li, such as graphite (either natural or synthetic), coke, and calcined organics; Li alloys such as Lixe2x80x94Al, Lixe2x80x94Mg, Lixe2x80x94In, Lixe2x80x94Alxe2x80x94Mn alloys; and metallic Li. In such instances, a charge voltage of about 2.8 V and a discharge voltage of about 1.8 V will be given. The contemplated effect of improving cycle life performances becomes more significant when the carbon materials, among those active materials, are used for the negative active material. This is because the carbon materials are contrary in property to the Li alloys and metallic Li which, during charge and discharge, are likely to be accompanied by the growth of needlelike dendrites that could cause internal short circuits, and because the occurrence of sulfur slightly dissolved in the electrolyte solution to react with metallic Li or Li in any Li alloy at the negative electrode can be avoided, which otherwise results in the deposition on the negative electrode surface of a compound, such as Li2S (See, for example, binary phase diagrams for Lixe2x80x94S in xe2x80x9cBinary Alloy Phase Diagramsxe2x80x9d, American Society for Metals, Vol.2, (1986), p.1500) that could deactivate the negative electrode.
In the case where the aforementioned titanium complex sulfide is used as the negative active material, a lithium-containing transition metal oxide, such as LiCoO2, LiNiO2, LiMn2O4, LiMnO2, lithium-containing MnO2, LiCo0.5Ni0.5O2, LiNi0.7Co0.2Mn0.1O2, LiCo0.9Ti0.1O2, LiCo0.5Ni0.4Zr0.1O2 or the like, can be used as the positive active material. In such instances, a charge voltage of about 2.3 V and a discharge voltage of about 1.3 V will be given. Those batteries are generally assembled in a discharged state and can be brought to a dischargeable condition by first charging them, i.e., by allowing Li present in the positive active material to migrate into the negative active material. The use of titanium complex sulfide as the negative active material thus results in a marked improvement of charge-discharge cycle life characteristics. This is considered due to the reduced charge voltage whereby the reductive decomposition of the electrolyte is suppressed.